1. Field of the Invention
This invention relates to a multi-frequency signal receiving circuit in which the normal operation can be ensured even when signal frequencies constituting a signal arrive with a large arriving time difference, the operation start time is quickened, and any fluctuation of an operation delay time and a release delay time due to an input signal level shaft is minimized.
2. Description of the Prior Art
There are two types of prior art systems for receiving a multi-frequency signal. In one such system each channel output obtained by rectifying an output of each of the filters of a receiving circuit corresponding to received signal frequencies, i.e., information of each channel, is applied to a common control circuit in which the arrival of a signal is discriminated by duration of a predetermined number of channel outputs not shorter than a predetermined period of time to control each channel output circuit. That is, only information of each channel is applied to the common control circuit but information with respect to received signal energy is not applied. In another type of system an output obtained by rectifying or amplification-rectifying an output of an input signal amplifier directly without using such filters, i.e., information with respect to received signal energy, is applied to a common control circuit in which the arrival of a signal is discriminated by duration of the received signal energy not shorter than a predetermined period of time to control each channel output circuit. That is, only information of the received signal energy is applied to the common control circuit but information of each channel is not applied.
These prior art systems have the following respective disadvantages. In the first prior art system, assuming that a signal is constituted by two frequencies, it happens that one signal frequency arrives with a delay of a certain period of time after the arrival of the other signal frequency because signal propagation delay time differs in dependence on the frequency in the case of a long transmission line. Even if this arriving time difference is in the order of one second, the normal operation of the receiver is requested. When the two signal frequencies are received with such an arriving time difference, a long period of time for checking the existence of a predetermined number of channel outputs is required for preventing a malfunction of the receiver. When operating in such a manner, however, if the two signal frequencies arrive at the same time, the time period from the signal arrival to the receiver output delivery, that is, the operation delay of the receiver is inevitably much prolonged. Namely, this first prior art system has a disadvantage in that a long operation period of the receiver is inevitably required in order to prevent a malfunction of the receiver due to a signal constituted by signal frequencies arriving with different arriving times.
In the second prior art system, because the arrival of a signal is discriminated by duration of received signal energy not shorter than a predetermined period, if a click noise is caused immediately before arrival of the correct signal, the common control circuit may discriminate it as the arrival of the correct signal. Therefore, it happens that a crosstalk output of a channel other than channels of the signal frequencies constituting the correct signal may be derived as the output of the receiver. Further, when two signal frequencies arrive with different arriving times, because the common control circuit discriminates the arrival of the earlier propagated signal frequency as the arrival of the correct signal by its energy, the receiver will deliver an output regarding only the earlier arrived signal frequency as a receiver output. Also, at arrival of the later propagated signal frequency, the receiver will deliver as its outputs an output regarding the later arrived signal frequency and a crosstalk output of another channel in addition to the already delivered output regarding the earlier arrived signal frequency. Thus, in the second prior art system, the receiver causes a malfunction in the case where there is induced a click noise immediately before the signal, or the arriving time difference between the two signal frequencies constituting the signal is large, which results in impossibility of use in this system in such an environment.